1. Field of the Invention
The present invention relates to display construction, and more particularly to displays and methods for reducing thickness of the front glass to reduce depixelization in collimate and post diffuse type active matrix displays.
2. Description of the Related Art
For notebook liquid crystal displays (LCD), it is very desirable to make the display as thin and light as possible. In recent years, 0.7 mm thick glass substrates have been adopted by LCD display manufacturers to replace the previous standard of 1.1 mm thick glass. To reduce manufacturing costs, starting glass substrate size has continued to increase and is now approaching 1 m by 1 m. This creates severe manufacturing problems due to the sag of the glass when supported horizontally only along two side edges. For a 1 m by 1 m by 0.7 mm substrate, the sag would be about 108 mm (G. Meda, xe2x80x9cSupport Design for Reducing the Sag of Horizontally Supported Sheetsxe2x80x9d"" SID ""00 Digest, pp. 161-163).
Using thinner starting glass substrates also requires the expensive redesign of the robotic handlers on all the processing equipment. Therefore, it is less desirable economically to achieve thinner displays by using thinner starting glass substrates.
An alternative method of making a thin liquid crystal display for notebook applications based on a combination of lapping and polishing of the glass after the cell has been assembled has been described (See, e.g., H. Ohkuma, K. Tajima, and K. Tomiki, xe2x80x9cDevelopment of a manufacturing process for a thin lightweight LCD cellxe2x80x9d. SID ""00 Digest, pp. 168-169). As described in this publication, after assembling a cell with an 0.7 mm thick array glass substrate with an 0.7 mm thick color filter substrate, the two substrates are simultaneously thinned down to 0.5 mm so that the final cell thickness is about 1.0 mm. The bulk of the material (e.g.,  greater than 90%) is removed by lapping using rough abrasive compounds. Polishing is used to make both sides of the cell smooth using fine abrasive compounds. The use of lapping reduces the total process time. Generally, mechanical thinning assumes equal thinning for both the array glass and the color filter glass.
Other methods of thinning down LCD displays along with double sealing methods to protect the electrical bonding area during the thinning process have also been disclosed. (Japanese Patent numbers HO5-249422, HO5-249423, HO5-61011, S56-29251 and U.S. Pat. No. 5,766,493). In Japanese Patent numbers HO5-249422 and HO5-249423, a thinning process based on etching is described along with a double seal to protect the electrical bonding area. In Japanese Patent number HO5-61011 assigned to Rohm, a process is described where after sealing two substrates together with patterned transparent electrodes on the substrates, the glass substrates are thinned by lapping. After lapping, the liquid crystal is injected.
In Japanese Patent number S56-29261 assigned to Hitachi, after cell processing, both the glass substrates were thinned down to achieve the proper cell gap. U.S. Pat. No. 5,766,493, assigned to LG Electronics, is directed at using a combination of both etching and polishing to thin down an LCD array.
Thinning the array substrate makes the attachment of the electrical connections to the array more difficult, especially if chip-on-glass (COG) bonding is used. The trend in the industry for portable displays is to use COG attachment instead of the more traditional tape carrier package (TCP). For TCP, the driver chip is on a flex circuit which is bonded to the substrate. COG results in a thinner display module and reduced cost (see H. Nishida, K. Sakamoto, H. Ogawa, H. Ogawa, xe2x80x9cMicropitch connection using anisotropic conductive materials for driver IC attachment to a liquid crystal displayxe2x80x9d, IBM J. Res. Develop. Vol. 42 No. xc2xe May/July 1998, pp. 517-525).
With COG, the single crystal silicon driver chip is bonded directly to the array glass substrate and an anisotropic conducting adhesive or film (ACA or ACF) is used to make electrical connections between the driver chip and the array plate (R. Aschenbrenner, A. Ostmann, G. Motulla, E. Zakel, and H. Reichl, xe2x80x9cFlip chip attachment using anisotropic conductive adhesives and electroless nickel bumpsxe2x80x9d, IEEE Trans on components, packaging, and manufacturing technology-part C, Vol. 20, No. 2, April 1997 pp. 95-100). The bonding process uses heat and pressure to attach the chip. The thermal expansion coefficient of the silicon chip and the glass substrate are different, so after bonding when they cool down to room temperature, there is a thermal expansion mismatch stress.
The stiffness of the Si is much greater than that of the glass and since they are of approximately the same thickness (about 0.7 mm for the glass and 0.5 mm for the Si chip) the glass tends to bend more to relieve the stress than the Si driver chip does. This leads to problems in maintaining the liquid crystal cell gap uniformity near the driver chips and in maintaining good electrical contact between the driver chip and the array substrate. Reducing the thickness of the array substrate glass increases these difficulties since the glass then tends to bend further.
An additional problem with the thinner glass is increased breakage during the module processing. Reducing the substrate thickness from 0.7 mm to 0.5 mm has resulted in an 55% increase in breakage. The breakage is generally due to chips along the edge of the substrate, which results in cracks in the glass.
LCD displays, typically include two glass plates or substrates, e.g., a color filter (CF) substrate and a thin film transistor (TFT) array substrate. Cracking concerns are especially important for the TFT array substrate since this substrate extends beyond the CF substrate on at least two sides to provide space for the electrical interconnection to the gate and data lines on the TFT array substrate.
In a collimate and post diffuse (CPD) type display, see e.g., U.S. Pat. No. 4,171,874 and Zimmerman et al. in xe2x80x9cViewing-angle enhancement system for LCDs,xe2x80x9d in SID ""95 Digest, pp. 793-796, or McFarland et al. in xe2x80x9cSPECTRA VUE(trademark) viewing angle enhancement system for LCDs,xe2x80x9d in Asia Display ""95 Digest, pp. 739-742, a highly collimating backlight is used in combination with a diffuser on the viewer side of the display (typically, the exterior side of the color filter substrate, outside the front polarizer).
For low resolution collimate and post diffuse type displays (e.g., less than about 100 pixels per inch (ppi)), the degree of collimation required is determined largely by the contrast ratio versus angle of transmission of the twisted nematic (TN) liquid crystal. An exit diffuser tends to redirect some of the light which is not normal to the display back into the normal direction which reduces the contrast ratio normal to the display. For example, with a TN display which may include a peak contrast ratio of 307:1, when an exit diffuser is added and the input light is collimated to xc2x110 degrees full width half maximum (FWHM) in air, the peak contrast ratio is reduced to 270:1.
For input collimation of xc2x115 degrees FWHM, the peak contrast ratio is 251:1 and for input collimation of xc2x120 degrees FWHM, the peak contrast ratio is 223:1. So as the degree of input collimation is reduced with the same exit diffuser, the peak contrast ratio is reduced. A contrast ratio of greater than 200:1 is desirable, but in actual use conditions the contrast ratio is greatly reduced by ambient light reflection from the front surface of the display, especially the anti-glare coating (which diffuses light into all directions to avoid direct reflections of lighted regions), if one is present.
With a high resolution collimate and post diffuse type display (greater than about 100 ppi), inadequate collimation of the backlight can result in unacceptable depixelization of the display. The depixelization occurs because the area which is illuminated on a diffuser by the light transmitted by one subpixel expands into the area illuminated by the light from another subpixel. The degree of depixelization depends on the distance from the diffuser to the color filters, which are on the inside of the cell, (e.g., the distance equals the thickness of the front substrate, e.g., the color filter substrate, and the thickness of the polarizer) and the degree to which the backlight is collimated. The more the backlight is collimated, the lower the overall efficiency of the backlight. So it is desirable to limit the amount of collimation required as much as possible.
Referring to FIG. 1, the depixelization can be characterized by a width w of the subpixel image on a diffuser 2 (e.g., at 50% peak intensity) divided by subpixel pitch p. The depixelization is given by
Depixelization=(subpixel opening size+2xc3x97spread)/pixel pitch, 
where spread is the thickness of the substrate (and polarizer) times the tangent of the angle of light of 50% peak intensity in the substrate relative to the substrate normal.
A depixelization ratio of less than about 1.6 is probably acceptable. For example, with a 120 ppi vertical stripe triad display, the horizontal subpixel pitch p is about 71 microns with an opening 4 (subpixel opening) in a black matrix 6 of about 56 microns wide. If input light 5 is collimated to xc2x120 degrees FWHM in air, CF substrate 7 is 0.7 mm thick and a front polarizer 11 is 150 microns thick, both of which have an index of refraction of about 1.5, the depixelization is approximately equal to (56+2(850 tan(20/1.5)))/213=2.15. Note that in the horizontal direction the pixel pitch is 3 times the subpixel pitch. A thin film transistor substrate 9 (0.7 mm thick) is also shown with a polarizer 13 (150 microns thick).
Therefore a need exists for devices and methods which provide reduced depixelization from high resolution collimate and post diffuse type liquid crystal display devices. Additionally, a need exists for devices and methods, which provide an improved process window for electrical connections to an array substrate and which reduce the probability of breaking of display substrates for a portable display when the substrates are reduced in thickness.
A display device and method for fabricating a liquid crystal cell are disclosed. A liquid crystal cell includes a first substrate, and a second substrate attached to the first substrate. The first substrate has a same thickness as the second substrate. The first substrate is lapped at a first rate while concurrently lapping the second substrate at a second rate, which is different from the first rate. The first substrate and the second substrate are thinned to different thicknesses. The thinner of the first and second substrates is provided on a viewer side of a collimate and post diffuse type liquid crystal cell to reduce depixelization.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.